Absorption cycle heat pump model for control design

Abstract

Abstract-Heat pumps have recently received increasing interest due to green energy initiatives and increasing energy prices. In this paper, a nonlinear dynamic model of a singleeffect LiBr-water absorption cycle heat pump is derived for simulation and control design purposes. The model is based on an actual heat pump located at a larger district heating plant. The model is implemented in Modelica and is based on energy and mass balances, together with thermodynamic property functions for LiBr and water and staggered grid representations for heat exchangers. Model parameters have been fitted to operational data and different scenarios are simulated to investigate the operational stability of the heat pump. Finally, this paper provides suggestions and examples of derivation of lower order linear models for control design.

Analysis of decentralized control for absorption cycle heat pumps

“…The cycle consist of four main components: an evaporator, an absorber, a generator, and a condenser. Thermodynamic state points are numbered in the figure (adopted from [13], [17]) and characterize the properties of the internal state of a liquid LiBr-water solution (1-6), water vapor (7,10), and liquid water (8,9) and the external state of a hot water source (11,12), cooling water (13)(14)(15)(16) and chilled water (17,18). Further, two valves and a solution pump provide the required circulation of solution and refrigerant internally in the absorption cycle.…”

“…In this paper, we use the so-called Relative Gain Array (RGA) analysis ( [14], [15]) to investigate the best input/output pairings for control of a generic heat pump model derived in a companion paper [16]. The analysis is carried out on a simulation model of an actual industrial absorption heat pump situated at Sønderborg Fjernvarme (SFJV); a district heating plant in the southern part of Denmark.…”

Coefficient of performance optimization of single-effect lithium-bromide absorption cycle heat pumps

“…Instead, the heat transfer is modeled using 1-dimensional dynamic staggered grid flow models in a finite volume representation with N volume elements (uniformly distributed) and N +1 flow elements in between the volumes. Details of the staggered grid models can be found in [12], along with descriptions of the valve and pump models.…”

“…Further, the model is implemented in the Modelica modeling language. A detailed derivation of the model was given in [12] and a summary is provided in the following.…”

“…Note, however, that the tuning of such existing feedback control loops is beyond the scope of the present paper. The results are based on a nonlinear dynamic simulation model described in [12], where the model parameters are adjusted to measurements from the actual heat pump and give a fair accuracy in the operating area. A dynamic model is used to also capture the effect of changes in set-points on potential LiBr crystallization issues.…”